Why would it be impossible to suck a liquid into your mouth using a straw on the moon?

There is no such thing as a “suction” force.

When you suck water up a straw, you are not really pulling it up the straw. You are just moving some of the air in your mouth out of the way, creating a slightly lower pressure. Since air is pressing down on the water in the cup, it’s forced up the straw until the pressure differential is equalized.

You could perform an instructive experiment, but I can tell you what will happen. The experiment is this:

Get yourself a cup with a lid with an integral straw.

Fill it with water.

Now, using aquarium caulk, seal the margins between the lid and the cup and between the straw and the lid. If there is a small air hole in the lid, seal that too. Wait for the caulk to cure.

Have a sip. Try again. Try harder. Not too hard, though, you may injure yourself.

If air is not allowed to flow into the cup, there is no force to push water up the straw. It might move a little as atmospheric pressure caves in the lid, but that’s it. You could put an industrial vacuum pump on the straw and it won’t get a drop (not unless you count water vapor). It can’t, because there is no such force as “suction,” only atmospheric pressure rushing in to fill the void.

On the moon (outside a pressurized habitat) there is no air pressure, so straws don’t work.

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